1. Field of the Invention
This invention is generally related to mechanical mounts and methods of mechanical mounting, and in particular, to methods and apparatus for mounting optical components, such as lasers, in a manner that controls stress development and that also permits high heat flow from the components.
2. Relevant Background
High power lasers and laser amplifiers having very high beam quality are difficult to make because a number of factors degrade performance. One such factor is heat deposition within the laser material resulting from imperfect conversion of pump radiation power into laser output power. Most frequently it is not absolute temperature rise that is of concern but, instead, spatial gradients within the material. It is well known that parameters, such as the refractive index, are temperature dependent, and consequently temperature gradients lead to refractive index gradients, which in turn degrade the performance of the laser or amplifier. Heat loads and poor mechanical mounting techniques also cause stresses in the materials that degrade performance by introducing wavefront distortions.
In an attempt to address problems associated with stresses generated during mounting, considerable efforts have been expended within the laser industry in finding ways to mount laser rods and slabs that optimize optical performance. Compounding the problem is the fact that a relatively large amount of heat must be removed from the material during mounting, and as a result, implemented mounting techniques typically provide a good heat conduction path to a heat sink or cold plate where the heat is removed through convection, conduction, or radiation. A further complication is that laser radiation must be extracted from the rod or slab through apertures and at least some of the laser material must be exposed to permit pump light to enter the material for absorption.
A number of techniques have been conceived to mount round laser rods, and these techniques are aimed at permitting good heat flow and at addressing stress production. One technique is disclosed by Guch in U.S. Pat. Nos. 4,594,716 and 4,601,038. Another technique is described by Rapoport et al. in U.S. Pat. No. 5,331,652. A third technique that is more specifically aimed at mounting round laser rods for low stress is disclosed by Sumida in U.S. Pat. No. 5,272,710 and involves mounting the rod in a transparent sleeve with an elastomer providing a mechanically compliant conduction path between the rod and sleeve. Common to all these techniques is the assumption of round laser rods. Such circular geometries are attractive in part because of the ease of fabrication, but, unfortunately, these mounting techniques are not particularly well suited for slabs having a rectangular shape (or not being round in cross section).
Slab geometries are attractive for several reasons and are particularly useful in the generation of high optical output powers. First, they provide at least one rectangular flat surface through which pump light can enter. Second, with uniform pumping, slab geometries promote one-dimensional temperature gradients. Third, they provide a method to “zig-zag” a laser beam within the medium. The latter provides for a way to extract energy efficiently with good beam quality. In order to maximize the advantages inherent in such slab geometry, it is, however, important to design the mounting and cooling arrangements very carefully.
Some efforts have been made in the laser industry to address some of the challenges associated with mounting of slab geometries. For example, a number of issued patents describe methods of mounting slabs that incorporate a gas or liquid flow channel between the source of pump radiation and the slab, e.g., U.S. Pat. Nos. 4,378,601; 4,468,774; 4,881,233; and 4,563,763. These methods or approaches have several drawbacks and do not adequately meet the needs of the laser industry. First, these methods assume the presence of a flowing cooling liquid and, therefore, preclude operating with a passive heat disposing mechanism. Second, these methods are often susceptible to the depositing of unwanted contaminants on the surface of the slab that may degrade the performance of the laser.
A somewhat more attractive solution may involve an arrangement in which passive heat spreaders and radiators are used without flowing coolants, such as cooling gas or liquid, being provided as an intrinsic part of the construction. For example, U.S. Pat. No. 4,949,346 discloses a method to sandwich a slab between transparent conductive heat sinks which also act as guides to transport pump light to the slab. However, the described method fails to solve the problem of removing large amounts of heat from the assembly. Also, the method includes bonding the slab to the conductive heat sinks, which does not work well at high thermal loads since it promotes stresses within the laser slab. An alternative method applicable to end-pumped slabs is disclosed in U.S. Pat. No. 6,014,391. In this alternative method, curved surfaces are used to concentrate pump light, and absorbing materials are attached to the ends of the slab. However, this approach is very complex and requires fabrication of curved surfaces, which is more difficult and expensive than the fabrication of flat surfaces.
None of these methods effectively provides for heat conduction directly to a high thermal conductivity material. Generally, heat conduction takes place through an intermediate material, such as elastomers, glasses, or crystals. These materials have thermal conductivities that are generally 10 to 1000 times or more lower than metals and consequently provide a far higher thermal resistance than metals.
Some attempts have been made utilizing amalgams of mercury. Unfortunately, the use of toxic mercury carries with it potential health hazards, and additionally, these efforts have applied only to round geometries. More significantly, these efforts do not effectively reduce stresses on the laser material because the mounting techniques teach providing a fixed volume of amalgam, which typically will result in transferring stresses to the laser rod when parts of the assembly undergoes thermal expansion as is explained below with reference to FIGS. 1A to 1D.
Of course, removing heat from confined spaces is a problem faced in industries other than the optics or laser industry. For example, removing heat is often a concern in the operation of integrated circuits. Numerous methods have been devised to conduct heat from integrated circuits, and several of these methods involve the use of liquid metals. Such heat removal methods are disclosed, for example, in U.S. Pat. Nos. 6,665,186; 6,748,350; 6,281,573; 6,656,770; 5,658,831; 5,572,404; and 5,561,590. The general purpose of these heat removal techniques differs in fundamental ways from the purpose in optics in that the desire is to permit the conducting medium to expand and contract rather than the heat source. The integrated circuit chip itself (i.e., the heat generator) is typically rigidly bonded on one side to a mount, and the compliant conducting material fills the space between the other side of the chip and a heat sink. As a result of such construction, the conducting material may deform as a result of relative motion between the chip and heat sink generating stresses internal to the chip. For at least this reason, bonding techniques used for integrated circuits do not address the needs for “stress-free” mounting of optics such as lasers.